Stall

Why an aircraft stalls?

An aircraft stalls when the streamlined/laminar airflow (or boundary layer) over the wing’s upper surface, which produces lift, breaks away from the surface when the critical angle of attack is exceeded, irrespective of airspeed, and becomes turbulent, causing a loss in lift (i.e., the turbulent air on the upper surface creates a higher air pressure than on the lower surface). The only way to recover is to decrease the angle of attack (i.e., relax the backpressure and/or move the control column forward).

What properties affect an aircraft’s stall speed?

An aircraft will stall at a constant angle of attack (known as the critical angle of attack). Because most aircraft do not have angle of attack indicators (except “eyebrows” on some electronic flight instrument system displays), the pilot has to rely on airspeed indications. However, the speed at which the aircraft will stall is variable depending on the effects of the following properties.

1. Weight

a. Actual weight b. Load factor, g in a turn c. Effective weight/center of gravity position

2. Altitude

3. Wing design/lift

4. Configuration

5. Propeller engine power

How does the stall speed vary with weight?

The heavier the aircraft, the higher is the indicated speed at which the aircraft will stall.

If an aircraft’s actual weight is increased, the wing must produce more lift (remember that the lift force must equal the weight force), but because the stall occurs at a constant angle of attack, we can only increase lift by increasing speed. Therefore, the stall speed will increase with an increase in the aircraft’s actual or effective weight.

The stall speed is proportional to the square root of the aircraft’s weight.

What wing design areas delay the breakup of airflow (stall)?

1. Wing slots are the main design feature that delays/suppresses stall speed. A slot is a form of boundary layer control that reenergizes the airflow to delay it over the wing from separating at the normal stall speed. The wing therefore produces a higher coefficient of lift (CL) and can achieve a lower speed at the stall angle of attack.

2. Lower angle of incidence and a greater chamber for a particular wing section, e.g., wing tips.

What changes the aircraft’s angle of attack at the stall?

The movement of the center of pressure point at the stall causes a change in the aircraft’s angle of attack. Normally, a simple swept or tapered wing is designed so that the center of pressure will move rearward at the stall. This is so because the stall normally is induced at the wing root first, where the center of pressure is at its furthest forward point across the wing span. Therefore, the lift produced from the unstalled part of the wing, toward the tips and therefore aft, is behind the root with an overall net result of the center of pressure moving rearward, which results in a stable nose-down change in the aircraft’s angle of attack at the stall.

What happens to the stall speeds at very high altitudes, and why?

The stall speed increases at very high altitudes, which the jet aircraft is capable of, because of

1. Mach number compressibility effect on the wing. At very high alti-tudes, the actual equivalent airspeed (EAS) stall speed increases because the Mach number compressibility effect on the wing disturbs the pressure pattern and increases the effective weight on the wing, resulting in a higher EAS stall speed.

2. Compressibility error on the IAS/ASI(R). The compressibility correc-tion that forms part of the difference between the indicated airspeed (IAS) and airspeed indicator (reading) [ASI(R)] (which is uncorrected) and equivalent airspeed (EAS, which is IAS corrected for compressibility and position instrument error) is larger in the EAS to IAS/ASIR direction due to the effect of the Mach number, resulting in a higher IAS stall speed.

What is a superstall?

A superstall also may be referred to as a deep stall or a Locked in stall condition, which, as the name suggests, is a stall from which the aircraft is unable to recover. It is associated with rear-engined, high-T-tail, swept-wing aircraft, which because of their design tend to suffer from an increasing nose-up pitch attitude at the stall with an ineffective recovery pitching capability. The BEA Trident crash in 1972 at Slough, England, is probably the most famous and tragic outcome of a superstall. A superstall has two distinct characteristics:

1. A nose-up pitching tendency

2. An ineffective tailplane

The nose-up pitching tendency at the stall is due to:

a. Near the stall speed, the normal rooftop pressure distribution over the wing chord line changes to an increasing-leading-edge peaky pattern because of the enormous suction developed by the nose profile. At the stall, this peak will collapse.

b. A simple virgin swept- or tapered-wing aircraft will stall at the wing tip first (if the wing has not been designed with any inboard stall properties) mainly due to the greater loading experienced, leading to a higher angle of incidence that causes the wing tip to stall. Because of the wing sweep, the center of pressure moves inboard to a point where it is forward of the center of gravity, therefore creating an increasing pitch-up tendency.

c. The forward fuselage creates lift, which usually continues to increase with incidence until well past the stall. This destabilizing effect has a significant contribution to the nose-up pitching tendency of the aircraft. However, these phenomena themselves are not exclusive to high-T-tail, rear-engined aircraft and alone do not create a superstall. For a superstall to occur, the aircraft will have to be incapable of recovering from the pitch-up tendency at the stall.

An ineffective tailplane makes the aircraft incapable of recovering from the stall condition, which is due to :

a. The tailplane being ineffective because the wing wake, which has now become low-energy disturbed/turbulent air, passes aft and immerses the high-set tail when the aircraft stalls. This greatly reduces the tailplane’s effectiveness, and thus it loses its pitching capability in the stall, which it requires to recover the aircraft. This is so because a control surface, especially the elevator, requires clean, stable, laminar airflow (high-energy airflow) to be aerodynamically effective.

What systems protect against a stall?

Stall warner’s and stick pushers. Stall warners are either an artificial audio warning and/or a stick shaker, which usually are activated at or just before the onset of the prestall buffet. Stick pushers are normally used only on aircraft with superstall qualities and usually activate after the stall warning but before the stall, giving an automatic nose-down command. Both systems normally receive a signal from an incidence-measuring probe.

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